Amazon EC2
Amazon began renting computation by the hour: a machine could be taken for a day and returned, with nothing bought.
Why it matters
Computation turned from a capital expense into a running cost, and a research group stopped depending on owning a cluster.
Before this, an experiment needing a hundred machines required either your own hardware or access to a supercomputing centre. EC2 removed the barrier. The consequence for AI was cumulative rather than immediate: by the early 2010s renting GPUs was routine, and a startup could train a model without a server room.